The investigators will refine a murine model of human granulocytic ehrlichiosis (HGE), and then use this model to investigate issues of host immunity to infection. They have developed an experimental mouse model of HGE -- mice infected with the HGE agent develop transient neutropenia and anemia (similar to the human illness). In this project they will further characterize and optimize the murine model of HGE so that it can be used to better understand host immunity and pathogenesis. They will use mice and a human isolate of HGE (NCH-1) to examine the kinetics of HGE infection and the distribution of the HGE agent in various murine organs during the early- (up to 1 month) and late- (1 to 6 months) stages of infection. They will then determine how the murine genotype, age, the experimental method of HGE inoculation (tick or needle challenge) and HGE isolate influence the course of infection and disease over a period of up to 6 months. Finally, they will delineate the humoral and cellular immune responses to HGE (using whole-cell lysates of purified HGE) that develop during the course of murine infection and determine whether mice can be actively or passively immunized (by immunizing mice with HGE- whole-cell-lysates or the passive transfer of anti-HGE-serum) against infection. After optimizing these elements of the murine model of HGE, they will then identify the immunogenic HGE-antigens that are recognized by antibodies elicited during infection - in order to begin to understand host immunity to infection. They will probe whole-cell lysates of HGE with sera from HGE-infected mice to identify the immunogenic antigens, and use an HGE genomic expression library to clone the genes that encode the immunogenic proteins. These proteins will be expressed in recombinant form and used to delineate the antigen-specific B and T cell responses that develop during infection. Finally, they will identify the specific immunogenic proteins that elicit protective and/or disease-modulating immune responses during murine infection. The development and characterization of a murine model of HGE, including the HGE-specific host immune response to infection, will further our understanding of this disease, and provide information on host immunity to infection.